In an ejector cycle, a compressor 101, a refrigerant radiator 102, an ejector 103 and a gas-liquid separator 104 are connected in a circuit through refrigerant pipes, as shown in FIG. 9. Also, a pressure-reducing device 105 such as a fixed throttle and a refrigerant evaporator 106 are connected between the ejector 103 and the gas-liquid separator 104 through a bypass pipe. A liquid refrigerant separated from a gas refrigerant in the gas-liquid separator 104 is sucked into a low pressure inlet 108 of the ejector 108 through the bypass pipe. This kind of ejector cycle is for example disclosed in Unexamined Japanese Patent Publication No. JP-A-11-37577.
In a vehicle air conditioning apparatus, it is generally required to meet a wide range of load change depending on use conditions such as from a low load condition during a dehumidifying operation in winter to a high load condition during a cooling operation in summer. The ejector 103 has a fixed nozzle 107 and the diameter of an outlet of the nozzle 107 is fixed irrespective to a flow rate of the refrigerant. If the above ejector cycle is employed to the refrigerating cycle of the vehicle air conditioning apparatus, it is difficult to cope with such wide load change by the ejector 103.
To cope with the wide load change in the cycle, it is proposed to use an ejector having a variable nozzle 112 shown in FIG. 10, for example, in Unexamined Japanese Patent Publication No. JP-A-5-312421. In a variable nozzle 112, a diameter of a nozzle outlet is changeable by a needle valve 111 according to the load condition and the flow rate of the refrigerant.
When an outside temperature is low and a flow velocity of cooling air is high, a refrigerant pressure at a high pressure side is lowered in the refrigerating cycle. Furthermore, when a temperature of air supplied to the evaporator is high or the amount of the air is large, that is, when a load of the refrigerant evaporator is high, a refrigerant pressure at a low pressure side is increased. As a result, a pressure difference between the high pressure side and the low pressure side is decreased. In the vehicle air conditioning apparatus, for example, the above circumstance is likely to occur when an inside air is circulated while an outside air temperature is low.
Incidentally, in a refrigerating cycle having an expansion valve, even under the above circumstance, the refrigerant pressure at the high pressure side is increased and the refrigerant pressure at the low pressure side is decreased by reducing the diameter of the expansion valve, as shown in FIG. 11. By this, a refrigerant flow rate is maintained at or increased to an optimal rate. Accordingly, air can be blown from the evaporator with a predetermined desired temperature.
On the other hand, in the ejector cycle, the refrigerant is sucked into the low pressure inlet of the ejector from the evaporator by using a loss energy produced while decompressing the refrigerant at a nozzle portion of the ejector. By this, a flow rate of the refrigerant passing through the evaporator is maintained. When the pressure difference between the high pressure side and the low pressure side is decreased under the above circumstance, a flow rate of the refrigerant is decreased at a high pressure inlet of the ejector through which a high pressure refrigerant is introduced in the ejector. As a result, the loss energy used to suck the refrigerant from the evaporator is insufficient. Therefore, it is difficult to facilitate the flow of refrigerant in the evaporator. As a result, the temperature of air blown from the evaporator is increased.
Furthermore, if the refrigerant pressure in the low pressure side is reduced while the refrigerant pressure in the high pressure side is not increased as shown in FIG. 12, the refrigerant evaporator is likely to be frosted. For example in the dehumidifying heating operation, it is difficult to produce air at a predetermined temperature (for example, 3 degree Celsius) from the evaporator.